Pyrocarbon and process for depositing pyrocarbon coatings

ABSTRACT

Pure unalloyed pyrocarbons having wear resistance suitable for use in pyrocarbon-coated heart valve components and having mechanical properties, such as flexural strength and toughness, superior to commercial silicon-alloyed pyrocarbons are deposited in fluidized bed coaters. Coating conditions are carefully controlled so as to maintain a precise bed size within a defined coating enclosure which will assure a substantially constant surface deposition temperature that in turn assures deposition of homogenous unalloyed pyrocarbons having these improved mechanical properties.

BACKGROUND OF THE INVENTION

This invention relates to pyrocarbon and to processes for depositingpyrocarbon coatings on substrates in a fluidized bed. More particularly,the invention relates to processes for depositing pure, unalloyedpyrocarbon having improved physical properties on substrates beinglevitated in a coating region along with a bed of small particles andalso to pyrocarbon having improved mechanical properties heretoforeobtained only in silicon-alloyed pyrocarbon structures.

BACKGROUND OF THE INVENTION

Pyrolytic carbon, i.e. pyrocarbon, coatings were developed some 30 yearsago as coatings for minute spheroids of nuclear fuel to be used in hightemperature gas-cooled fission reactors, and substantial effort wasexpended in attempting to improve the properties of these pyrocarbonsfor such purposes. Of particular interest was the mechanical strengthand the stability of the pyrocarbon under high level neutron radiation.Examples of U.S. patents illustrative of such pyrocarbon coatings andprocesses for depositing these coatings include U.S. Pat. No. 3,325,363(Jun. 13, 1967) and U.S. Pat. No. 3,547,676 (Dec. 15, 1970).

Subsequently, it was unexpectedly discovered that pyrocarbon of aspecific character, i.e., having relatively high density of at leastabout 1.5 gm/cm³, having an apparent crystallite size of about 200 Å orless and having high isotropy, exhibited remarkably good properties foruse in devices that would have direct or indirect contact with thecirculatory system of humans because such pyrocarbon was outstandinglyinert and did not give rise to thrombosis. As a result, such pyrocarboncoatings have become the materials of choice for prosthetic heart valvesand other components of apparatus for interconnection with thecirculatory system of humans. U.S. Pat. No. 3,685,059 is indicative ofsuch pyrocarbons.

Although such pyrocarbons were expected to have reasonably good hardnessand structural strength, it has uniformly been felt that, in order toassure adequate structural strength required for many biomedicalpurposes, it was necessary to add a carbide-forming alloying element,such as the metalloid silicon which would form silicon carbide. It hasthus become common to employ up to about 20 weight percent of siliconwhich was found to provide the alloyed pyrocarbon with hardness,wear-resistance and overall structural strength sufficient to meetestablished criteria; such use of silicon is pointed out in the '059patent at Column 5, which also describes other carbide-forming elementswhich could alternatively be used as additives. Most frequently, such analloying element was used in an amount from about 1 and 6 atom percent,based upon total atoms of carbon plus the element. Typically, pyrocarboncoatings for heart valve components which have been used in commercialheart valves in the United States over the past decade have includedfrom about 5 to about 12 weight percent of silicon. The addition ofsilicon was generally carried out by adding methyltrichlorosilane to theupwardly flowing stream of a mixture of hydrocarbon and inert gas thatwas being used to deposit the coating. It is believed that allFDA-approved pyrocarbon coatings for heart valve components include atleast about 5 weight percent silicon carbide for the purpose ofincreasing hardness and wear resistance.

Unfortunately, silicon carbide is not as nonthrombogenic as pyrocarbon.Although it has been acceptable for the pyrocarbon coatings to include aminor amount of silicon carbide as an alloying agent, because siliconcarbide does exhibit some thrombogenicity, the biocompatibility ofpyrocarbon coatings with the bloodstream could be improved if heartvalve components could instead be coated with pure, unalloyedpyrocarbon. Significant processing improvements would also occur in thepyrocarbon deposition process if it were not necessary to handle apotentially corrosive substance, such as methyltrichlorosilane or thelike; such an elimination would reduce both capital costs and operatingcosts. Not only would the cost of this additional raw material beeliminated, but the cost of the tubing network and control necessary toinject or supply precise amounts of a chlorosilane into the levitatinggas stream would be eliminated. It would also reduce many of the safetyprecautions necessary in such a coating operation as well as in thecleanup steps upon the completion of a coating run.

Studies were carried out in about 1974 relating to the character ofpyrocarbon deposited at various temperatures in expanding beds fromgaseous atmospheres containing different volume percents of a variety ofhydrocarbons, and the results were published in a U.S. Government(Advanced Research Projects Agency) report No. GA-A13350--CarbonResearch--Final Technical Report for the Period Jan. 1, 1974 to Dec. 31,1974. Because such a growing bed would fairly soon collapse, coatingsonly about 1 mil in thickness were generally obtained. Unalloyedpyrocarbons deposited in such study using a low volume % of propane at asight port temperature of 1350° C. were reported to have a density ofabout 1.46 g/cm³, a crystallite size of about 20 Å and a DPH hardness ofabout 153. The hardness number is a standard Diamond Pyramid Hardnessmeasured with a 50 gram load. By increasing the propane to 60 volume %at this temperature, a carbon density of about 1.97 was obtained forthese thin pyrocarbon samples, with a crystallite size of about 36 Å anda hardness of about 219 DPH. In contrast by adding 8 weight % silicon topyrocarbon of about 1.97 density the DPH hardness rises dramatically toabout 295. On the other hand when the sight port temperature is raisedto about 1475° C. (bed temperature about 1400° C.), the densitydecreases to about 1.46, the hardness to about 92 DPH and thecrystallite size to about 26 Å. Young's modulus, fracture strength andhardness were all shown to generally rise with increasing density, andit was also shown that denser (and harder) carbons provided higher wearresistance. One conclusion of the study was that hardness and thus wearresistance of carbons increased with increasing density which wouldfavor the lower temperature isotropic carbons; however, it was felt thata significant reduction in wear rate was more easily obtainable byalloying the pyrolytic carbon with silicon, which path has generallybeen followed.

Therefore, improved processes which would eliminate, or at least vastlylower, the amount of alloying silicon carbide needed to providepyrocarbon with desired strength, resistance to cracking and hardnesshave long been desired.

SUMMARY OF THE INVENTION

It has now been found that, by depositing pyrocarbon under verycarefully controlled conditions in a fluidized bed, pure, unalloyedpyrocarbon coatings having high fracture toughness, high strength, andhigh strain-to-failure, along with adequate hardness, can be provided.Because of the absence of any silicon carbide, such pure, unalloyedpyrocarbons have improved biocompatibility and outstandingnonthrombogenicity.

It was surprisingly found that such pure, unalloyed pyrocarbons of theforegoing characteristics can be consistently deposited in a fluidizedbed if conditions can be carefully controlled within a fairly narrowtemperature range above about 1325° C. and below or at about 1450° C. bycarrying out the pyrolytic decomposition of a hydrocarbon gas that isbeing supplied, usually as a part of a levitating gas-coating mixturecontaining at least about 40 volume percent of inert levitating gas.Using propane, for example the preferred range is from about 1375° C. toabout 1425° C. By maintaining a truly constant bed surface area, it hasbeen found that the actual bed temperature at the surfaces wheredeposition is occurring can be precisely controlled and will result inisotropic, pure pyrocarbon of desired hardness and structural strengthbeing consistently deposited in a fluidized bed coating zone. Thus,pyrocarbon coatings of adequate thickness and structural characteristicsto permit their use in prosthetic devices, particularly heart valveprostheses, can be formed without the inclusion of a silicon alloyingagent.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view, partly in section, of a fluidized bedapparatus for depositing pyrocarbon onto objects being levitated inassociation with a bed of particles, which is exemplary of an apparatusthat can be used to carry out processes for depositing pyrocarboncoatings embodying various features of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A fluidized bed coating apparatus 20 is illustrated in FIG. 1 whichincludes a furnace 22 having an outer cylindrical shell 24 within whicha coating enclosure is located. The coating enclosure is generallyseparated from the cylindrical outer shell of the furnace by a layer ofinsulation 25 and is defined generally by a thin sleeve or tube 26 incombination with a lower insert 28. The lower insert 28 mates with thebottom end of the thin tube 26 and provides the coating enclosure with aconical bottom surface 30; it includes a central passageway 32 thatextends vertically upward therethrough and is preferably coaxial withthe thin tube 26 which is of circular cross-section. Although thereshould be no size limitations on such coaters, of general interest arecoaters having tubes 26 not greater than about a foot in diameter andparticularly those having an interior diameter of about 6 inches orless. The levitating atmosphere flows upward through this passageway 32to enter the coating enclosure.

The upper end of the furnace tube 24 includes a plate 33 which centersthe coating tube 26 therewithin, and an exit passageway 36 from thecoating enclosure is defined by an upper insert 34 which extendsdownward into the furnace and narrows the outlet opening somewhat, whichinsert has a frustoconical lower surface 37. The remainder of themixture of hot levitating and coating gases leaving the fluidized bedcoater pass through this upper exit passageway 36 and flow to anappropriate vent. A particle feeding device 40 is mounted generallyabove the fluidized bed coater 20 and is designed to feed minuteparticles 41 into the coating enclosure at a desired constant orvariable rate. The particles from the feeder 40 enter the coater throughan entrance conduit 42 which extends downward through the wall of theupper insert 34, terminating at the frustoconical surface 37. A suitableinduction or alternating current heating device 44 is provided, as wellknown in this art. It is disposed in surrounding relationship to thefurnace tube 24 in order to heat the active deposition region of thecoating enclosure, together with the small particles that make up thebed and the objects being levitated in the bed, to bring them to thedesired deposition temperature for pyrolytic coating.

The coating operation is carried out by establishing a levitated bed ofminute particles, submillimeter in size, which is maintained in thelower region of the coating enclosure; this region is referred to as theactive deposition region, or as the coating zone. As illustrated, theactive deposition region generally occupies the conical region definedby the upper end surface 30 of the lower insert; however, depending uponthe size of the bed employed, it may extend upward a few inches into thecylindrical section thereabove. Once the bed is established and broughtup to coating temperature by applying an appropriate amount ofelectrical power to the heating unit 44, one or more objects to becoated, such as annular valve bodies or occluders for prosthetic heartvalves, are appropriately loaded into the bed through the upper exitpassageway 36 shortly before coating is ready to begin. Once in the bed,these objects are levitated and supported among the fluidized particlesby the upwardly flowing gaseous stream, where they are likewise heatedto raise their temperature to the desired coating temperature. Thetemperature within the coating enclosure is appropriately set andmonitored as is well known in this art, as by using opticaltemperature-measuring devices, such as a sight-port pyrometer thatfunctions with a viewing port in the furnace wall (not shown). It hasbeen recognized that the temperature read by such a sight-port pyrometerwill be somewhat above the actual bed temperature, which can beaccurately read electrically (e.g. by thermocouples); however, it hasnow been found that the bed temperature can change substantially whenchanges in the bed surface area occur, which can dramatically affect theproperties of the pyrocarbon being deposited.

The upwardly flowing gas stream, during the time of coating, is made upof a mixture of an inert fluidizing gas plus a gaseous hydrocarbon, asdescribed hereinafter. Illustrated is a source of hydrocarbon 46 whichis provided with a flow-regulating valve arrangement 48 alongside asource 50 of inert gas, for example argon, helium or nitrogen, which islikewise equipped with a suitable flow-regulating valve arrangement 52.These two gas sources flow into a common line 54 which leads to thevertical passageway 32 in the lower insert 28.

The total flow of levitating-coating gas upward through the coatingenclosure is regulated so that the fluidized bed occupies an activedeposition region which generally fills the conical region at the top ofthe insert 28 and may extend a few inches upward into the cylindricaltube 26 for larger beds. The upward flow of the gaseous stream throughthe central passageway causes a generally toroidal flow pattern to beestablished within the lower region of the fluidized bed, which isdefined generally by the conical surface 30 of the insert 28, with theminute particles traveling upward in the central region and thendownward generally along the outer perimeter of this toroidal flowpattern.

The particles may be chosen so as to have a density greater than thedensity of the pyrocarbon being deposited in the bed. The particles areabout 1,000 microns (μm) or less in size, and preferably, the originalbed is constituted by particles not greater than about 850 microns. Theparticles added during the coating run preferably have an average sizenot greater than about 400 microns. Generally particles between about300 and about 425 microns are added during the coating run, and theaverage size of the particles withdrawn over the coating run is greaterthan about 500 μm.

To remove particles from the bed, an exit conduit 56 is provided havinga circular access opening of a suitable diameter in its upper end. Theconduit 56 can extend through the entire lower insert 28, or it can beshorter in length and be received in a coaxial counterbore provided atthe upper end of a drilled passageway where it will protrude from theconical surface 30 of the insert. The exit conduit 56 may open straightup or be capped at its upper end and have an exit hole in its sidewallas earlier described in this art. The conduit 56 may be proportioned sothat its upper end terminates within the vertical region defined by theconical surface 30, and preferably, the conduit 56 protrudes from theconical surface 30 a maximum vertical distance of less than about 1 inchfor a coater having an interior diameter of about 4 inches or less. Theexit conduit 56, which extends into the coating region, is formed of asuitable refractory material, such as graphite or mullite.

Particles being removed from the fluidized bed, which enter the conduit56 through such an access opening, fall downward by gravity to adischarge conduit 60 leading to a collection chamber 62 where they arereceived in a graduated cylinder 63 or the like so that the total amountand/or volume of particles having been removed can be determined at anytime either visually or through the use of a load cell 65. Thecollection chamber 62 is pressurized with inert gas from a suitablesource 66 which is precisely controlled through the use of aflow-regulating valve arrangement 68 that is suitably set by a controlarrangement discussed in detail hereinafter. The flow of inert gas outof the pressurized collection chamber upward through discharge tube 60and the exit conduit 56 serves both as a purge flow to preventsubstantial quantities of dust from falling into the collection chamberand also to precisely regulate the rate of removal of particles from thefluidized bed. By increasing the gas pressure within the collectionchamber 62, the rate of upward flow of inert gas will increase, with aresultant decrease in the rate, or a total cessation, of withdrawal ofparticles from the bed. Decrease of gas pressure, continuously orperiodically, increases rate of withdrawal of particles over time.Feeding particles at a constant rate and withdrawing particles at arelatively constant rate should permit the amount of particles in thebed to be maintained at a precise predetermined level throughout acoating run.

It has been found that maintaining the surface area of the fluidizedparticle bed within a coating enclosure in the region where the coatingdeposition is occurring is a most important parameter in preciselycontrolling the rate of deposition and the character of the pyrocarbonbeing deposited. Measurement of the pressure difference (ΔP) across thefluidized bed so as to detect changes that occur is one effective way ofsensing even small changes in the overall surface area of the bed. Bymonitoring the pressure difference across the bed, i.e. between a pointat a lower region in the bed or just below the bed and a point above thebed, to detect changes which occur and then responding to such detectedchanges from the desired target value to compensate for such, precisecontrol of the pyrocarbon being deposited throughout a coating run isachieved.

Such monitoring is accomplished in the illustrated apparatus byemploying a pair of dedicated, pressure-sensing ports in the region ofthe coating enclosure. In the illustrated embodiment, an upper pressuresensing port 70 is provided in the form of an elongated passageway whichextends through the upper insert 34, and a lower pressure-sensing port72 is provided via a long passageway in the lower insert 28. The lowerport 72 is preferably located in the lower one-half of the volumetricregion of the bed or below the bed, and it is more preferably located inthe lower 25% of the volume of the bed. The upper port 70 and the lowerport 72 are respectively connected via tubing 74, 76 to a pressuretransducer 78 for measuring the pressure at these ports and comparingthe two pressures measured to determine the difference between the twomeasured pressures. If desired, optional, visually-readable gauges canbe included. Although a pressure transducer 78 is preferred, othersuitable pressure-measuring devices, such as manometers, canalternatively be used. To keep the ports and the tubing clear of dust,carbonaceous material and/or particles, an appropriate slow purge flowof inert gas (not shown) is maintained through both port systems. Purgeflows for each of the ports 70, 72 are constant, with purge gas beingappropriately injected into the tubings 74, 76. An initial pressuremeasurement is taken at the beginning of each coating run that takes therespective purge flow into consideration, and it serves as a referencepoint for that coating run.

A signal from the pressure transducer 78 is sent through a suitable line84 to a control unit 86. The control unit 86 compares the signal beingreceived with the values programmed into its memory to see if thedesired pattern is being maintained, and if deviation is detected, thecontrol unit 86 instigates appropriate adjustments. Adjustments to thesurface area of the bed are made by changing the rate at whichhydrocarbon is fed to the bed as a part of the fluidized gas flow, or bychanging the rate at which particles are supplied to and/or withdrawnfrom the coater. The amount of hydrocarbon being supplied to the coatingzone may be changed within ±10% of its initial value without measurablyaffecting the pyrocarbon properties; however, such change will have avery significant effect upon the rate of deposition. Therefore, if it isdesired to maintain a constant bed surface area by controlling the flowrate of hydrocarbon, the control unit 86 is connected by a suitable line90 through which a signal, preferably electric, is transmitted to thecontrol valve 48, which determines the rate of flow of hydrocarbonthrough the inlet conduit 54.

It is generally convenient to set the particle feeder 40 to feedparticles of a desired size into the coater 20 at a substantiallyconstant rate, and a control line 88 leading to the feeder 40 isprovided for this purpose. Therefore, when it is desired to control thebed surface area by controlling the number of particles in the coatingzone, it is preferred to alter the rate of removal of particles from thefluidized bed. Details of such control of the bed surface area are setforth in U.S. Pat. No. 5,284,676 (Feb. 8, 1994), the disclosure of whichis incorporated herein by reference. The load cell 65 under the beaker63 into which the removed particles fall is connected by suitablecontrol line 94 to the control unit 86 and provides a signal which isindicative of the total weight of beaker plus particles. The controlunit compares the weight versus time and thereby determines whether theprecise amount of particles intended has been removed during each presetinterval of time, for example, each 30 or 60 seconds. As a result ofthis precise reading which is accomplished by using the load cell 65,the control unit 86 will normally cause minor changes to be made (eitherby varying the rate of flow of nitrogen upward through the particleremoval conduit 60 or by varying the off-on intervals if a high gas flowis used to periodically halt particle removal) whenever needed tomaintain the desired rate of withdrawal of particles throughout thecoating run.

Should a fluidized bed become too small, the actual bed temperature willdecrease and the bed may no longer be able to properly levitate theobjects being coated, in which case there will be danger that the entirebed will collapse. It has been found that if the surface area of the bedis maintained substantially constant, unalloyed pyrocarbon of certaindesired physical properties can be deposited by carrying out pyrolyticdeposition under certain parameters. By monitoring the pressure acrossthe bed, it is possible to promptly detect a decrease in the bed surfacearea as soon as it begins, and the control unit 86 will promptly andautomatically effect countermeasures so as to return the surface area ofthe bed to its desired size, as by slowing the rate of removal ofparticles from the bed. As a result, the surface area of the bed willpromptly gradually increase until the desired size of bed surface areais re-established. On the other hand, if the surface area of the bedwere to grow excessively large, the actual bed surface temperature wouldincrease, the characteristics of the pyrocarbon being deposited wouldalso undesirably change, and the rate of deposition of carbon on theheart valve bodies and occluders would slow. Such is also undesirable;accordingly, it is considered important to prevent the bed surface areafrom growing oversized. Accordingly, when the control unit senses achange in the pressure difference across the bed indicative of anundesirable increase of the surface area above the target size,countermeasures are likewise promptly automatically effected to increasethe rate of removal of particles from the bed until the targetdifferential pressure is re-established.

Any suitable electronic controller can be used, such as one commerciallyavailable from Inotek/Analog Devices, a UDC 9000 Multi-Pro fromHoneywell, or an IBM 386 or 486 computer and Control E.G. software. Bymonitoring for changes in pressure difference indicative of changes inbed size and immediately effecting compensating changes, as necessary,to return the bed to the target size, the rate of deposition and theproperties of unalloyed pyrocarbon being deposited are very carefullycontrolled throughout an entire run of several hours in length. Coatingsof unalloyed pyrocarbon having the desired improved mechanicalproperties can be uniformly deposited during an individual coating runso as to have a minimum thickness of at least about 0.1 mm of pyrocarbonand preferably at least about 0.2 mm in key regions that will besubjected to wear in heart valves or the like. The thickness of thedeposit will be in part dependent upon the overall size of thesubstrate, and the pyrocarbon may be at least 0.5 millimeter thick andoften about 1 mm or more in thickness when larger objects are beingcoated. It is possible to repeatedly coat objects with unalloyedpyrocarbon coatings having these improved physical properties and havingcarefully controlled precise thickness to within about 1 mil (0.001inch). This is particularly valuable when coating parts for prostheticheart valves wherein precision in achieving uniform physical propertiesand tolerances is extremely important and thus commercially veryvaluable.

Although a pressure transducer 78 is the preferred device for monitoringthe pressure above the bed and either just below or in a lower region ofthe bed, other suitable pressure sensing devices can be employed asindicated hereinbefore. It has also been found to be satisfactory tohave the pressure transducer 78 simply monitor atmospheric pressureoutside the coating apparatus 20, instead of monitoring the pressurejust above the particle region of the bed in the enclosure, becauseunder these circumstances the pressure therein is essentiallyatmospheric. Monitoring atmospheric pressure and comparing it with thepressure in the lower region of the bed monitored via the conduit 72 andfeeding a signal representative of the difference between these twovalues to the control unit 86 provides an adequate alternative way ofmaking measurements to detect changes in the bed size for whichimmediate compensation is effected in order to precisely obtain thecoating characteristics and thicknesses desired in a pyrocarbondeposition run.

Pyrocarbon is, by definition, deposited by the high temperaturepyrolysis of a carbon-containing substance; it is thus required that thesubstrate upon which deposition occurs be stable at the fairly hightemperatures to which it will be subjected during pyrolysis. Substratesof commercially available isotropic artificial graphite, such as thatsold as AFX-5Q and AFX-5Q-10W by the POCO Graphite Company, of Decatur,Tex., are preferred. However, other artificial graphites having adensity between about 1.7 and about 2.1 g/cm³ which are close toperfectly isotropic, e.g. having an isotrophy of about BAF 1.1 or less,can be used.

Hydrocarbons are the preferred carbon-containing substances forpyrolysis, and hydrocarbons having 1 to about 5 carbon atoms, forexample propane, propylene, isobutane, pentane, ethane and methane willpyrolyze at temperatures between about 1000° C. and about 1325° C.Particular hydrocarbons that are employed may somewhat vary thecharacteristics of the pyrocarbon deposited, as can the carbon contentof the coating atmosphere. The latter is generally controlled bycontrolling the concentration of the hydrocarbon gas, usually using amixture of a hydrocarbon gas, i.e. coating gas, and an inert levitatinggas, such as nitrogen, helium or argon or a mixture thereof, although100% methane may be used. It has been found that the pyrocarboncharacteristics desired can be perhaps best obtained by employing amixture of a hydrocarbon, preferably having a carbon chain lengthbetween 2 and 4 carbon atoms, in combination with at least about 40volume percent of an inert gas and preferably at least about 60%. Thepreferred hydrocarbon gas which is employed is propane, propylene,isobutane or ethane or a mixture thereof. For example, a mixture ofnitrogen and propane containing from about 25% to about 60 volume % ofpropane may be used.

Whereas previously it had been thought the hardness of pyrocarbonfollowed the density so that pyrocarbons having sufficiently highhardness to provide the necessary wear resistance for heart valvecomponents and the like would be found only in pyrocarbons having adensity of about 2 grams per cm³ or above, it has now been unexpectedlyfound that there is a region of coating condition parameters wherecoating of unalloyed pyrocarbon occurs having a density as low as about1.85 grams per cm³ wherein these carbons exhibit hardness of about 220DPH or higher, which have properties that are adequate for wearresistance. Even more surprisingly, these carbons have been found toexhibit toughness which is as much as about 50% higher thansilicon-alloyed pyrocarbons of comparable hardness and to have flexuralstrength up to about 20% greater than similar silicon-alloyedpyrocarbons. As a result, these unalloyed pyrocarbons are considered tobe more than just the equivalent of silicon-alloyed pyrocarbonscontaining between about 5 and about 12 weight % silicon, thus makingthem particularly valuable in this respect for biomedical applicationswhere silicon is undesirable from the standpoint of potentialthrombogenicity. Moreover, from a fracture mechanic's point of view, anincrease in toughness of 20% would have been a significant advance, andthe observed 50% increase is remarkable indeed.

Unalloyed pyrocarbons having these improved physical features, can bedeposited in fluidized beds when the sight-port temperature is set at arange between about 1325° and about 1450° C. However, from an overallstandpoint the sight-port temperature is preferably maintained betweenabout 1350° and about 1425° C., more preferably at about 1375°-1425° C.,and the bed is controlled so the surface temperature is held at asubstantially constant value by carefully maintaining a constant bedsurface area. By substantially constant is meant that the bed surfacetemperature does not deviate more than about 25° C. and preferably notmore than about 15° C. from the original set point at the beginning ofthe particular coating step.

It is likewise important that a relatively high surface area to volumeratio be maintained in the coating region so that isotropic pyrocarbonhaving these desired physical properties will be deposited, and it hasbeen found that the ratio of the bed surface area in square centimetersto the volume of the coating region in cubic centimeters should bebetween about 10:1 and about 50:1, and preferably between about 20:1 andabout 40:1. The bed volume may vary within a coating enclosure of givensize with substantial changes in the flow rate of the levitating streamthrough the bed, because a faster rate of flow will further expand thebed. For example, a 300 gram charge of pyrocarbon coated zirconium oxideparticles of the size range indicated above will have a surface area ofabout 12,060 square centimeters; for such a charge, it may be desirableto employ a flow of the coating-levitating gas of at least about 15liters per minute up to a flow of about 20-25 L/min in a 3.5 inch I.D.coater. At flow rates between about 15-20 L/min, the bed will occupy avolume of about 475 to 520 cubic centimeters. A charge of about 700grams of zirconium oxide particles of this type will have a bed surfacearea of about 28,140 square centimeters, and with a similar flow rate ofgas, the bed may occupy a volume of about 680 to about 775 cubiccentimeters in a similar coater.

The volume of flow per minute of the mixture of coating gas andfluidizing gas will have an influence upon obtaining the desired ratioof bed surface area to volume in a coater; however, the flow rate isalso regulated to supply an approximate amount of carbon to the coatingzone per unit time. Generally, for coaters having an interior diameterof up to about 6 inches, it is found to be advantageous to use a flow ofbetween about 0.1 liter and about 0.5 liter, per square centimeter ofthe cross-sectional area through the cylindrical section of the coatingenclosure and preferably between about 0.2 and about 0.4 L/min/cm². Forexample, for a coater having a 3.5-inch interior diameter (about 63.6square centimeters of cross sectional area), it is advantageous to use aflow rate which is between about 15 and about 25 liters per minute ofthe coating-fluidizing gas mixture, at standard temperature andpressure, taking in consideration the size of the bed. For example, fora bed of about 300 gms of ZrO₂ particles as described hereinbefore, aflow rate of the levitating stream of about 15-20 L/min may bepreferred. With this flow rate, the volume percentage of coating gas isadjusted to deliver a relatively constant amount of carbon, i.e. thecarbon content, to the coating zone per unit time. Therefore, when aparticular coating operation runs effectively at 40% propane, acomparable atmosphere composed of about 30% isobutane-70% nitrogen or ofabout 90-100% methane might be used.

Even within these parameters, it has been found important that theconditions be fairly precisely maintained throughout the coating run sothat pyrocarbon having a substantially homogenous crystallite size andisotrophy will be deposited. In addition to controlling the bed size forthe purpose of maintaining the desired ratios, it is important that thebed size be maintained within strict limits because even small changesin bed size, as indicated hereinbefore, result in distinct changes inthe actual temperature at the surface of the substrates where depositionis occurring; such changes in temperature not only affect the rate atwhich carbon is being deposited but they can affect the physicalcharacteristics of the carbon itself. By precisely controlling thesevarious parameters within the stated limits, it has now been foundpossible to consistently deposit unalloyed pyrocarbon having outstandingmechanical properties. Young's modulus is usually measured to provide arepresentative modulus of elasticity for a material such as pyrocarbon,and pyrocarbon having a Young's modulus (which is the ratio of simpletension stress to the resulting strain parallel to the tension, reportedin psi×106) between about 3.9 to 4.1 is routinely obtained. The modulusof rupture for bending is the maximum stress per unit area that aspecimen can withstand without breaking when it is bent, and it isusually reported in psi×10³ for these materials. This is sometimesreferred to as flexural strength, and unalloyed pyrocarbons made underthese parameters can have a modulus of rupture of about 58 to 64 orhigher. Hardness was earlier discussed, a pyrocarbon should have a DPHof at least about 200 in order to be considered to be adequate toprovide the desired wear resistance over the life of a heart valvecomponent. These pyrocarbons are preferably deposited so as to have ahardness of at least about 220 DPH and more preferably a DPH of about230 or higher, e.g. up to about 250. They should also be isotropic,having a BAF of not greater than about 1.1 and preferably between 1.0and about 1.075. Another important physical property for pyrocarbon isthe measurement of strain to failure; whereas silicon-alloyed pyrocarbongenerally had a strain-to-failure of about 1.2%, these unalloyedpyrocarbons have a strain-to-failure of about 1.3 to 1.4%, which is asignificant increase for this property. K_(IC) is a value which issometimes referred to as fracture toughness; it is a measure of theforce required to propagate an existing small crack in pyrocarbon and isusually reported in units of Ksi (1000 psi)×√in, or alternatively asMPa(√m). It has been found that unalloyed pyrocarbons are produced usingthe aforementioned temperature range and conditions that have a fracturetoughness as much as about 50% higher than present commercial isotropicpyrolytic carbon coatings; these commercial coatings that contain analloying amount of about 7 weight % of silicon have a K_(IC) of about1.05. Thus, not only is the need for silicon-alloying in pyrocarbons tobe used for biomedical purposes eliminated but at the same time, a 50%improvement in toughness is achieved--a truly surprising and totallyunexpected result.

The following are examples of the operation of a fluidized bed coatingapparatus in a manner so as to carry out processes for depositingpyrocarbon embodying various features of the present invention.

EXAMPLE 1

When a coating run is ready to begin, a predetermined bed size isestablished by loading a precise amount of pyrocarbon-coated ZrO₂particles to provide the desired amount of surface area and setting thetemperature and the flow rates of inert gas (nitrogen) and hydrocarbon(propane), as well as setting a constant rate of feeding particles and anominal rate of withdrawal of particles. This is all conveniently donethrough the control unit via control lines 91 and 93, which respectivelylead to the adjustable valves 52 and 68. A fluidizing flow of an inertgas, such as nitrogen, is established upward through the coater 20 bysetting the valve 52 via the control line 91 to supply nitrogen at adesired rate of flow from the source 50, which may be a pressurized tankor the like. A suitable charge of particles is then added to the coater,through the upper end, to create a fluidized bed having the desiredamount of surface area. For example, in a coater having an internalcoating enclosure diameter of about 31/2 inches, one may begin with acharge of about 250 to about 700 grams of pyrocarbon-coated zirconiumdioxide particles having sizes greater than about 325 microns but lessthan about 850 microns. The particles of this initial bed are preferablypyrocarbon-coated versions of the uncoated zirconia particles that willsubsequently be fed into the bed during coating. Uncoated particleswhich have a density of about 5.37 grams per cm³ and a preferred sizeranging from about 300 microns to about 425 microns (with an averagesize of about 360 microns) are loaded into the particle feeder 40. For atypical coating operation in a coater of about this size, about 20 to 40heart valve orifice rings are added to the bed; such rings, when coated,will serve as valve bodies for prosthetic heart valves. Exemplaryorifice rings generally have the form of short tubes, having a height ofabout 0.7 cm., an I.D. of about 1.5 to 2.5 cm., and an O.D. of about 1.6to 2.6 cm. U.S. Pat. Nos. 5,152,785 and 5,192,309 show generallyrepresentative heart valves having valve bodies of this general type.The substrates to be coated are added after the particle bed has beenbrought up to its operating temperature of about 1400° C. using theinduction heating apparatus 44.

The coating apparatus is then operated until both the particles and theobjects reach the desired temperature. During this warmup period, thecontrol unit via the control line 93 causes a sufficient flow ofnitrogen to be maintained upward through the conduit 56, which has anentrance aperture in the form of about a 3/16 inch inner diameter openupper end, to prevent particle withdrawal. A purge flow rate, as high asabout 4 liters per minute, of nitrogen is maintained through the lowerpressure port 72 so as to prevent particles from entering this port. Theexit conduit 56 is located so that its open upper end is located about0.4 inch below the top edge of the conical surface 30 of the lowerinsert 28, wherein it is in about the middle of the bed. For alevitating gas flow of about 25 liters per minute through this coatingenclosure wherein the conical surface of the lower insert has a verticalheight of about 5 inches, when a 700 gram charge of coated ZrO₂particles is used along with 40 orifice rings, the bed may occupy avolume of about 700-800 cubic centimeters.

When a coating run is ready to begin, the control unit 86 causes thevalve 48 to be opened to its predetermined set point, as desired forthis particular run, and flow of a preferred coating gas, i.e. propane,is begun so that a mixture of it and the fluidizing gas that is alreadyflowing through the line 54 into the central passageway 32 is beingsupplied at the desired rates of flow. The flow rates of the inert gasand propane are set so that the flow of the fluidizing-coating gasmixture upward through the coating enclosure, measured at standardtemperature and pressure, is about 14 liters/minute of nitrogen andabout 7 liters/minute of propane, for a total flow of about 21liters/min. As soon as the coating run begins, the feeder 40 is causedto feed uncoated zirconia particles into the apparatus at a constantrate of about 2.0 grams per minute via the entrance passageway 42through which they fall to become a part of the fluidized bed. Becauseof their greater density, the small uncoated particles 41 quicklygravitate to a lower level in the bed. The control unit 86, via thevalve 68 and the control line 93, controls the amount of nitrogen whichflows upward through the particle discharge passageway 60 so thatparticles are withdrawn at an initial rate of about 7 grams per minute,which rate is maintained until the control unit detects changes areoccurring in the bed surface area. A purge flow of about 4 liters ofnitrogen per minute is maintained through the entrance passageway 42,flowing along with the falling zirconia particles, and a purge flow ofabout 1 liter of nitrogen per minute is maintained through the tubing 74leading to the upper pressure sensing port 70.

The pressure difference between the lower port 72 and the upper port 70is monitored throughout the coating run by a pressure transducer 78(e.g. a commercially available Sensotec Model Z or a SETRA Model C239).An output signal from the transducer which is representative of thedifference in measured pressures is transmitted to the electroniccontroller 86, and it is compared, for example, each 15 seconds, with aprogram that is representative of the desired bed size. The control unit86 thus monitors the pressure differential, and, as soon as changes aredetected, it causes appropriate adjustments to be made in the rate ofwithdrawal of particles from the coating enclosure. This monitoring andthe instigation of countermeasures to changes in bed surface areacontinues throughout the entire coating run, which may typically takebetween about two and about four hours. For example, when an increase inpressure differential is detected above the target value which isindicative that growth in the bed surface area has occurred, the controlunit 86 acts to compensate for this growth by removing particles at aslightly faster rate.

The exemplary coating operation is carried out for about 180 minutesunder conditions so as to initially maintain desired bed surface areathroughout the entire coating run. At the conclusion of the coating runand cool-down, the coated articles are examined and the pyrocarbon isfound to be of high quality and to have a precise uniform thickness ofsubstantially 0.010 inch, having a tolerance within about 0.001 inch ofthe desired value.

The pyrocarbon deposited has a hardness of about 235 DPH, and it isconsidered to be satisfactory from a standpoint of providing adequatewear resistance for heart valve components. The flexural strength isabout 64×10³ psi, and the modulus of elasticity is about 3.9×10⁶ psi.The toughness is excellent, showing a K_(IC) of about 1.58. Thestrain-to-failure is about 1.4%. These unalloyed pyrocarbon-coatedcomponents are considered to be well suited for use as valve bodies inmechanical heart valves.

EXAMPLE 2

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1400° C. and a mixture of about 40 volume %propane and about 60 volume % nitrogen at a flow rate of about 20 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 500 grams is employed along with about 10 flat graphiterectangular slabs which are provided as the substrates for coating toallow precise measurement of carbon properties. Coating is carried outfor a sufficient time to deposit a pyrocarbon coating having a thicknessof about 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 235DPH and a density of about 1.89 gm/cm³ and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 63.2×10³ psi, andthe modulus of elasticity is about 3.9×10⁶ psi. The toughness isexcellent, showing K_(IC) of about 1.6. The strain-to-failure is about1.38%. The properties of these unalloyed pyrocarbon-coated substratesare such that components, such as valve bodies and occluders, made underthese conditions are considered to be well suited for use as valvebodies in mechanical heart valves.

EXAMPLE 3

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1400° C. and a mixture of about 40.5 volume% ethane and about 59.5 volume % nitrogen at a flow rate of about 20liters per minute. A charge of pyrocarbon-coated zirconium oxideparticles of about 500 grams is employed along with about 10 flatgraphite rectangular slabs which are provided as the substrates forcoating to allow precise measurement of carbon properties. Coating iscarried out for a sufficient time to deposit a pyrocarbon coating havinga thickness of about 0.01 inch; the coatings are uniform and have atolerance within about 0.001 inch. The pyrocarbon deposited has ahardness of about 235 DPH and a density of about 1.91 gm/cm³, and it isconsidered to be satisfactory from a standpoint of providing adequatewear resistance for heart valve components. The flexural strength isabout 62.6×10³ psi, and the modulus of elasticity is about 3.9×10⁶ psi.The toughness is excellent, showing K_(IC) of about 1.59. Thestrain-to-failure is about 1.38%. The properties of these unalloyedpyrocarbon-coated substrates are such that components, such as valvebodies and occluders, made under these conditions are considered to bewell suited for use as valve bodies in mechanical heart valves.

EXAMPLE 4

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1400° C. and a mixture of about 30 volume %isobutane and about 70 volume % nitrogen at a flow rate of about 20liters per minute. A charge of pyrocarbon-coated zirconium oxideparticles of about 500 grams is employed along with about 10 flatgraphite rectangular slabs which are provided as the substrates forcoating to allow precise measurement of carbon properties. Coating iscarried out for a sufficient time to deposit a pyrocarbon coating havinga thickness of about 0.01 inch; the coatings are uniform and have atolerance within about 0.001 inch. The pyrocarbon deposited has ahardness of about 235 DPH and a density of about 1.95 gm/cm³, and it isconsidered to be satisfactory from a standpoint of providing adequatewear resistance for heart valve components. The flexural strength isabout 62.3×10³ psi, and the modulus of elasticity is about 3.9×10⁶ psi.The toughness is excellent, showing K_(IC) of about 1.58. Thestrain-to-failure is about 1.37%. The properties of these unalloyedpyrocarbon-coated substrates are such that components, such as valvebodies and occluders, made under these conditions are considered to bewell suited for use as valve bodies in mechanical heart valves.

EXAMPLE 5

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1400° C. and of about 100 volume % methaneat a flow rate of about 20 liters per minute. A charge ofpyrocarbon-coated zirconium oxide particles of about 500 grams isemployed along with about 10 flat graphite rectangular slabs which areprovided as the substrates for coating to allow precise measurement ofcarbon properties. Coating is carried out for a sufficient time todeposit a pyrocarbon coating having a thickness of about 0.01 inch; thecoatings are uniform and have a tolerance within about 0.001 inch. Thepyrocarbon deposited has a hardness of about 222 DPH and a density ofabout 2.07 gm/cm³, and it is considered to be satisfactory from astandpoint of providing adequate wear resistance for heart valvecomponents. The flexural strength is about 57.5×10³ psi, and the modulusof elasticity is about 4.0×10⁶ psi. The toughness is excellent, showingK_(IC) of about 1.58. The strain-to-failure is about 1.29%. Theproperties of these unalloyed pyrocarbon-coated substrates are such thatcomponents, such as valve bodies and occluders, made under theseconditions are considered to be well suited for use as valve bodies inmechanical heart valves.

EXAMPLE 6

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1425° C. and a mixture of about 40 volume %propane and about 60 volume % nitrogen at a flow rate of about 20 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 500 grams is employed along with about 10 flat graphiterectangular slabs which are provided as the substrates for coating toallow precise measurement of carbon properties. Coating is carried outfor a sufficient time to deposit a pyrocarbon coating having a thicknessof about 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 226DPH and a density of about 1.81 gm/cm³, and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 61.8×10³ psi, andthe modulus of elasticity is about 3.7×10⁶ psi. The toughness isexcellent, showing K_(IC) of about 1.58. The strain-to-failure is about1.38%. The properties of these unalloyed pyrocarbon-coated substratesare such that components, such as valve bodies and occluders, made underthese conditions are considered to be well suited for use as valvebodies in mechanical heart valves.

EXAMPLE 7

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1450° C. and a mixture of about 40 volume %propane and about 60 volume % nitrogen at a flow rate of about 20 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 500 grams is employed along with about 10 flat graphiterectangular slabs which are provided as the substrates for coating toallow precise measurement of carbon properties. Coating is carried outfor a sufficient time to deposit a pyrocarbon coating having a thicknessof about 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 208DPH and a density of about 1.83 gm/cm³, and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 54×10³ psi, andthe modulus of elasticity is about 3.5×10⁶ psi. The toughness isexcellent, showing K_(IC) of about 1.54. The strain-to-failure is about1.33%. The properties of these unalloyed pyrocarbon-coated substratesare such that components, such as valve bodies and occluders, made underthese conditions are considered to be well suited for use as valvebodies in mechanical heart valves.

EXAMPLE 8

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1375° C. and a mixture of about 40 volume %propane and about 60 volume % nitrogen at a flow rate of about 20 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 500 grams is employed along with about 10 flat graphiterectangular slabs which are provided as the substrates for coating toallow precise measurement of carbon properties. Coating is carried outfor a sufficient time to deposit a pyrocarbon coating having a thicknessof about 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 231DPH and a density of about 1.96 gm/cm³ and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 63×10³ psi, andthe modulus of elasticity is about 3.9×10⁶ psi. The toughness is good,showing a K_(IC) of about 1.18. The strain-to-failure is about 1.41%.The properties of these unalloyed pyrocarbon-coated substrates are suchthat components, such as valve bodies and occluders, made under theseconditions are considered to be well suited for use as valve bodies inmechanical heart valves.

EXAMPLE 9

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1350° C. and a mixture of about 40 volume %propane and about 60 volume % nitrogen at a flow rate of about 20 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 500 grams is employed along with about 10 flat graphiterectangular slabs which are provided as the substrates for coating toallow precise measurement of carbon properties. Coating is carried outfor a sufficient time to deposit a pyrocarbon coating having a thicknessof about 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 226DPH and a density of about 2 gm/cm³, and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 57×10³ psi, andthe modulus of elasticity is about 3.9×10⁶ psi. The toughness isacceptable, showing a K_(IC) of about 0.96. The strain-to-failure isabout 1.33%. The properties of these unalloyed pyrocarbon-coatedsubstrates are such that components, such as valve bodies and occluders,made under these conditions are considered to be adequate for use asvalve bodies in mechanical heart valves.

EXAMPLE 10

The pyrocarbon deposition process set forth in Example 1 is repeatedusing a temperature of about 1400° C. and a mixture of about 25 volume %propane and about 75 volume % nitrogen at a flow rate of about 15 litersper minute. A charge of pyrocarbon-coated zirconium oxide particles ofabout 700 grams is employed along with about 8 flat graphite rectangularslabs which are provided as the substrates for coating to allow precisemeasurement of carbon properties. Coating is carried out for asufficient time to deposit a pyrocarbon coating having a thickness ofabout 0.01 inch; the coatings are uniform and have a tolerance withinabout 0.001 inch. The pyrocarbon deposited has a hardness of about 240DPH and a density of about 1.85 gm/cm³ and it is considered to besatisfactory from a standpoint of providing adequate wear resistance forheart valve components. The flexural strength is about 69×10³ psi, andthe modulus of elasticity is about 3.9×10⁶ psi. The toughness isexcellent, showing a K_(IC) of about 1.69. The strain-to-failure isabout 1.5%. The properties of these unalloyed pyrocarbon-coatedsubstrates are such that components, such as valve bodies and occluders,made under these conditions are considered to be well suited for use asvalve bodies in mechanical heart valves.

Although the invention has been described with regard to a number ofpreferred embodiments, it should be understood that various changes aswould be obvious to one having the ordinary skill in this art may bemade without departing from the invention which is defined by the claimsappended hereto. Although heart valve components have been the subjectof many of the illustrations, it should be understood that pyrocarbonhaving these excellent mechanical properties can be used for many otherbiomedical applications as well as other applications where theinclusion of an alloying agent, i.e. a metal or metalloid, such assilicon, that forms a carbide, would be considered undesirable.Furthermore, although one significant advantage of the invention is theprovision of pyrocarbon having these outstanding mechanical propertiesand excellent wear resistance without the necessary inclusion of siliconor some other alloying agent to increase hardness, it should beunderstood that up to about 3 weight % of silicon might be included inthese pyrocarbons without adversely affecting the outstanding fracturetoughness which they exhibit; accordingly, such carbons would beconsidered to be equivalents of these pure unalloyed pyrocarbons.Likewise, although the discussion is centered upon the employment of asingle hydrocarbon for the coating gas, it should be understood that amixture of two or more hydrocarbons could be used if desired, andsimilarly a mixture of two or more inert gases could likewise beemployed.

Particular features of the invention are emphasized in the claims thatfollow.

What is claimed is:
 1. A process for coating prosthetic heart valve components with pyrocarbon having high fracture toughness, high flexural strength and high strain-to-failure without including 5 or more weight percent of silicon in such pyrocarbon, which process comprises,establishing a bed of particles in a coating enclosure by levitating said particles so as to create a region within said enclosure wherein coating with pyrocarbon will occur and levitating a plurality of substrates for heart valve bodies or occluders along with said particles in said coating region, said particles providing sufficient surface area that the ratio of surface area of said bed of particles measured in square centimeters to the volume of the coating region measured in cubic centimeters is at least 10 to 1, maintaining said particles and said substrates at a temperature between about 1350° C. and about 1425° C., employing a levitating gas stream which includes a hydrocarbon that will pyrolytically decompose at said temperature and may also include an inert gas but which is substantially devoid of any alloying agent and maintaining a continuous upward flow of said gas stream through said coating region at a rate of between about 0.1 and about 0.5 liter per minute per sq. cm. of cross section of said coating region, said hydrocarbon having a carbon chain length not greater than about 5 carbon atoms, continuing said continuous upward flow of said gas stream through said coating region for a length of time sufficient to deposit a uniform coating having a thickness of at least about 0.2 mm of substantially unalloyed pyrocarbon upon each of said substrates, and maintaining a substantially constant bed surface area within said coating region by adding additional particles thereto while removing therefrom pyrocarbon-coated particles which have grown in particle size as a result of deposition of pyrocarbon so that the actual bed temperature at surfaces where deposition is occurring remains within a temperature range of about 50° C., whereby said substrates remain in said enclosure and are coated with unalloyed pyrocarbon having a density of about 1.8 to about 2.0 g/cm³, a K_(IC) of at least about 1.2, a modulus of rupture for bending of at least about 58, a strain-to-failure of at least about 1.3%, and a Young's modulus of about 4.0×10⁶ psi or below.
 2. A process according to claim 1 wherein said pyrocarbon has a K_(IC) of at least about 1.54.
 3. A process according to claim 1 wherein said substrates are made of isotropic artificial graphite.
 4. A process according to claim 1 wherein an inert gas is present and constitutes at least about 40 volume percent of said stream.
 5. A process for making unalloyed isotropic pyrocarbon having high fracture toughness, high flexural strength and high strain-to-failure, which process comprises,establishing a bed of particles in a coating enclosure by levitating said particles so as to create a region within said enclosure wherein coating with pyrocarbon will occur and levitating a plurality of substrates to be coated along with said particles in said coating region, said particles providing sufficient surface area that the ratio of surface area of said bed of particles measured in square centimeters to the volume of the coating region measured in cubic centimeters is at least 10 to 1, maintaining said coating region at a set temperature above about 1325° C. and at or below about 1450° C., employing a levitating gas stream which includes a hydrocarbon that will pyrolytically decompose at said temperature and maintaining a continuous upward flow of said gas stream through said coating region at a rate of between about 0.1 and about 0.5 liter per minute per sq. cm. of cross section of said coating region, said hydrocarbon having a carbon chain length not greater than about 5 carbon atoms, continuing said continuous upward flow of said gas stream through said coating region for a length of time sufficient to deposit a uniform coating of a desired thickness of at least about 0.1 mm of pure unalloyed pyrocarbon upon each of said substrates, and maintaining a substantially constant bed surface area within said coating region by adding additional particles thereto while removing therefrom pyrocarbon-coated particles which have grown in particle size as a result of deposition of pyrocarbon, so that the surface temperature of said particles and said substrates remains substantially constant during said length of time, whereby said substrates remain in said enclosure and are coated with unalloyed pyrocarbon having such properties with a density of between about 1.7 and about 2.1 g/cm³, a K_(IC) of at least about 1.2, a modulus of rupture for bending of at least about 58, a strain-to-failure of at least about 1.3%, and a Diamond Pyramid Hardness (DPH) of between about 200 and
 250. 6. A process according to claim 5 wherein the amount of said particles is selected so that the ratio of the surface area of said bed of particles measured in square centimeters to the volume of said coating region measured in cubic centimeters is at least about 20 to
 1. 7. A process according to claim 6 wherein said ratio is between about 20 to 1 and about 40 to
 1. 8. A process according to claim 5 wherein said hydrocarbon is selected from the group consisting of propane, propylene, ethane, methane, butane and mixtures thereof.
 9. A process according to claim 5 wherein said gas stream is a mixture of said hydrocarbon and an inert gas devoid of any alloying agent, which inert gas constitutes at least about 40 volume percent of said mixture, and said flow rate is between about 0.2 and about 0.4 L/min/sq.cm.
 10. A process according to claim 9 wherein said hydrocarbon and the volume percent of said hydrocarbon are selected so that said pyrocarbon deposited is isotropic and has a hardness of at least about 220 (DPH), and an apparent crystalline size of at least about 30 Å.
 11. A process according to claim 10 wherein said isotropic pyrocarbon which is deposited has a Bacon Aniosotropy Factor (BAF) between 1.0 and about 1.1.
 12. A process according to claim 11 wherein said deposited pyrocarbon has a density of between about 1.8 and about 1.95 gm/cm³.
 13. A process according to claim 5 wherein said temperature is maintained between about 1350° C. and about 1425° C.
 14. A process for making substantially unalloyed pyrocarbon having high fracture toughness, high flexural strength and high strain-to-failure, which process comprises,establishing a bed of particles in a coating enclosure by levitating said particles so as to create a region within said enclosure wherein coating with pyrocarbon will occur and levitating a plurality of substrates to be coated along with said particles in said coating region, said particles providing sufficient surface area that the ratio of surface area of said bed of particles measured in square centimeters to the volume of the coating region measured in cubic centimeters is at least 10 to 1, maintaining said particles and said substrates at a temperature between about 1325° C. and about 1425° C., employing a levitating gas stream which is a mixture of a hydrocarbon that will pyrolytically decompose at said temperature and an inert gas and is substantially devoid of any alloying agent, which inert gas constitutes at least about 40 volume percent of said mixture, and maintaining a continuous upward flow of said gas stream through said coating region at a rate of between about 0.1 and about 0.5 liter per minute per sq. cm. of cross section of said coating region, said hydrocarbon having a carbon chain length not greater than about 5 carbon atoms, continuing said continuous upward flow of said gas stream through said coating region for a length of time sufficient to deposit a uniform coating having a thickness of at least about 0.1 mm of substantially unalloyed pyrocarbon upon each of said substrates maintaining a substantially constant bed surface area within said coating region by adding additional particles thereto while removing therefrom pyrocarbon-coated particles which have grown in particle size as a result of deposition of pyrocarbon so that the actual bed temperature at surfaces where deposition is occurring remains within a temperature range of about 50° C., and whereby said substrates remain in said enclosure and are coated with unalloyed pyrocarbon having a density of about 1.8 to about 2.0 g/cm³, a K_(IC) of at least about 1.2, a modulus of rupture for bending of at least about 58, a strain-to-failure of at least about 1.3%, and a Young's modulus of about 4.0×10⁶ psi or below.
 15. A process according to claim 14 wherein said pyrocarbon has a K_(IC) of at least about 1.54.
 16. A process according to claim 14 wherein said temperature is maintained at between about 1375° C. and about 1425° C.
 17. A process according to claim 14 wherein the amount of said particles is selected so that the ratio of the surface area of said bed of particles measured in square centimeters to the volume of said coating region measured in cubic centimeters is at least about 20 to
 1. 18. A process according to claim 17 wherein said hydrocarbon is propane, said levitating gas stream upward flow rate is between about 0.2 and about 0.4 liters per sq.cm. per minute, and said coating is continued to deposit a uniform coating of a thickness of at least about 0.2 mm on said substrates.
 19. A process according to claim 14 wherein highly isotropic pyrocarbon is deposited having a BAF not greater than 1.075 and a hardness of at least about 230 (DPH). 